In a light-sensitive layer of conventional electrophotographic sensitive materials inorganic photoconductive substances such as selenium, cadmium sulfide, zinc oxide and the like have been widely used. In recent years research has been in progress on the use of organic photoconductive substances and some are practically used for electrophotographic sensitive materials.
Generally speaking, organic photoconductive substances have advantages over inorganic photoconductive substances in higher transparency, lighter weight, better film-forming ability and flexibility and easier design of sensitive materials using proper sensitizing methods.
Fundamental properties required for electrophotographic sensitive materials include (1) high chargeability when exposed to corona discharge in the darkness, (2) less leakage, in the dark, of the charge obtained (decay in the dark), (3) rapid release of the charge when exposed to light (decay with light) and (4) little residual charge after exposure to light.
Many researches have been made on photoconductive polymers such as polyvinylcarbazole to be used as organic photoconductive substances. These compounds, however, are not satisfactory in film-forming properties, flexibility and adhesiveness and are difficult to fully meet the above mentioned fundamental requirements. On the other hand, electrophotographic sensitive materials excellent in mechanical properties can be obtained with organic low molecular photoconductive compounds if polymers excellent in film-forming properties, flexibility, adhesiveness, etc. are selected as binders, but it has been difficult to find compounds suitable to produce sensitive materials of high sensitivity.
Recently, electrophotographic sensitive materials in which a specific mono-azo compound or bis-azo compound is used as photoconductive substance are proposed, for example, in Japanese Patent Unexamined Publications (Kokai) No. 37543/72 and No. 119926/79. These compounds, however, are not sufficient to improve sensitivity of sensitive materials and further improvement of sensitivity has been desired.